Crossing the fault

A “floating” water transmission line is designed to survive up to 6.5 feet of earth displacement during an earthquake. Second of two articles.

VERY TIGHT FIT. In Fremont, Calif., two corrugated steel water lines (BDPLs 3 and 4) cross three traces — or strands — of the Hayward Fault at a 45-degree angle. At the most potentially damaging strand, Trace B, the San Francisco Public Utilities Commission is replacing BDPL 3 with a solution designed to withstand 6.5 feet of displacement. The new pipe will be fitted with specially fabricated ball and slip joints housed within an articulated concrete vault that will let the pipe expand, contract, and rotate safely during an earthquake. Construction is scheduled to begin in May 2012 and end in 2014. Map: San Francisco Public Utilities Commission

Procuring this 72-inch ball joint has taken three years. Standard practice requires ball joints to undergo hydrostatic pressure testing with no rotation, so the manufacturers, EBAA Iron Inc. and R&B Co., designed and fabricated a special apparatus to test for rotation while at 200 psi. Tests showed the ball joints can withstand the forces associated with fault displacement. Photo: San Francisco Public Utilities Commission

In October, engineers from around the world met in Niigata, Japan, to hear about the first-of-a-kind pipeline design that will allow the San Francisco Public Utilities Commission to provide drinking water within 24 hours of our seismically active region's next major earthquake.

The innovative solution we and our partners developed is one of the most critical reliability projects in the commission's Water System Improvement Program covered last month in this magazine (see “A regional plan for seismic security” on page 26 of the November issue). Two of our water system's largest transmission mains cross three traces — or strands — of the Hayward Fault, a 74-mile crack in the earth's crust east of the San Francisco Bay. Moving at a rate of 6 millimeters a year, the fault has caused, on average, a major seismic event every 140 years. The most recent was 143 years ago, so we're three years overdue.

Without improvements, a major earthquake would probably damage one or both mains, threatening the regional water system and our ability to deliver drinking water to our 2.5 million customers around the Bay Area.

A comprehensive hydraulic study showed we could satisfy post-earthquake water-delivery needs using a single, 78-inch pipeline. We decided to completely replace and enhance one of the Bay Division Pipelines (“BDPL 3” on the map on this page) where it intersects with the fault. In addition to being able to respond to the earth's movement at this critical juncture, the new pipeline will be protected by a unique structure. We'll also upgrade the second pipeline (“BDPL 4”) to minimize damage to the new, seismically reliable pipeline during an earthquake.

Over the last seven years, our biggest challenge has been devising a fault-crossing system for the new pipeline at Trace B, the fault's most troublesome trace, where 6.5 feet of horizontal fault displacement is expected. We evaluated nine concepts in two years, including the zigzag concept used for the Denali Fault crossing of the 48-inch Alyeska oil pipeline in Alaska that has withstood a 14-foot offset from a magnitude 7.9 earthquake, before combining elements from several into the final solution.

The design features two 72-inch ductile-iron ball joints that rotate up to 12 degrees and a slip joint with a 9-foot compression capacity. These improvements allow the pipe to accommodate fault displacement by rotating at the ball joints and compressing at the slip joint. The ball and slip joints are housed inside an underground articulated vault that protects the pipe from the surrounding soil.

The enclosure provides free space in which the pipe can compress and rotate. It will be constructed of reinforced concrete, will be 20 feet wide, 18 feet tall, and 305 feet long, and will be made up of 11 separate segments. Six-inch gaps between each allow the segments to move relative to each other as well as toward each other as the gap closes. Inside the vault, the pipe rests on special supports, called sliding supports, that allow for horizontal movement of the pipe.

With these improvements in place, the pipeline will be able to withstand a 6.5-foot displacement during an earthquake.

Engineering for an earthquake

Despite predictions that the next major earthquake could happen very soon, we adhered to the same rigorous design process that's in place for our other Water System Improvement Program projects: alternative analysis, conceptual engineering, and design submittals at the 35%, 65%, 95%, and 100% milestones.

We also performed physical tests and computer-modeling analyses to make sure our design would work as planned.

To test the articulated vault, the Large-Scale Lifelines Testing Facility at Cornell University constructed a one-tenth scale model that underwent earthquake simulations. Five tests were performed, and certain features were varied to perfect the final design. Additionally, we used Itasca International Inc.'s FLAC3D continuum modeling software to analyze how the vault would behave within the surrounding soils during an earthquake.

We used ANSYS Inc.'s engineering simulation software to perform extensive computer modeling of the pipeline's response to both fault displacement and ground shaking behavior during a seismic event. The ball and slip joints were included in these models.

We also performed a scour analysis of the second main to ensure potential leakage doesn't harm the seismically improved water main.

And although it could increase the cost of manufacturing the ball and slip joints by roughly 50%, we decided to order an extra ball joint and may order an extra slip joint as full-scale test specimens. This will allow for rigorous factory testing and ensure full compliance with our performance criteria.

Minimizing local impacts

The project site is confined to an 80-foot right of way in Fremont, Calif. It's at the intersection of two state highways and close to homes, apartment buildings, shopping centers, and a Native American burial site. An environmentally sensitive creek passes through it. The pipelines will remain in service during construction; each will be taken out of service just once. Construction will last for more than two years.

So in addition to seismic reliability and the usual project issues of cost, constructability, operations and maintenance, and permitting, our solution had to address environmental and community impacts.

To minimize traffic relocations and road closures, we'll install temporary bridges and a secant pile wall at Trace B, where the pipelines run under one of the state highways, to support the bridges while excavation and installation of the articulated vault and new pipeline occur below. Traffic relocations will occur here to allow for the construction and removal of the bridges, but otherwise interruptions on these major thoroughfares will be minimal.

As with any underground construction project, we're coordinating utility relocations with assets owned by others. The most significant involves improving a local water agency's 12- and 30-inch pipelines where they intersect with ours to ensure they don't fail during an earthquake. Most utility relocations, including this one, have been folded into our project and will be undertaken by our construction team.

The true test comes next year as the Hayward Fault crossing, the last of the Water System Improvement Program's 81 major projects, receives notice to proceed. We'll advertise the $55 million construction project in November. We're scheduled to complete the final program project by July 2016.

—Labonte directs the San Francisco Public Utilities Commission's Water System Improvement Program and Wong is a project engineer for the commission. E-mail them at wsip@sfwater.org.

To be awarded: February 2012

WEB EXTRA

To watch videos that show how San Francisco's earthquake-resistant pipeline design works, click here.